WO 97/05742-A1 describes a system that can be used for detecting infra-red (IR) radiation. The system can, for example, be used in a target-seeking missile. The document describes a switchable mirror which can be set in two different positions. In a first position, the mirror is not active. In this position, radiation from an observed scene, which goes in through an objective lens, can reach a detector array. In a second position of the mirror, radiation from the scene is prevented from reaching the detector array. Instead, radiation from a light emitting diode is reflected towards the detector array. The light emitting diode is used as a temperature reference. In this system, it takes a relatively long time to move the mirror from the first to the second position and vice versa.